1. Field of the Invention
The present invention relates to a tubular solid oxide fuel cell and more particularly, a fuel electrode-supported tubular solid oxide fuel cell whose support strength is without degradation of cell performance. Also, the present invention is concerned with a less costly method of fabricating a fuel electrode-supported tubular solid oxide fuel cell, which is presently the key technical challenge facing SOFC.
2. Description of the Prior Art
Up to date, great advance has been brought about in the development of fuel cells. Phosphoric acid fuel cell (PAFC) and molten carbonate fuel cell (MCFC) are now being developed for the application for power generating systems of 100 kW or greater. Although it was studied later than PAFC and MCFC, solid oxide fuel cell (SOFC), so-called, the third generation fuel cell, is expected to be in practical use in the near future, subsequent to PAFC and MCFC, by virtue of the recent rapid development of material technology. To this end, advanced nations have devoted tremendous effort to the fundamental research on SOFC and enlarged production scale.
Since the operating temperature of SOFC is in the range from 700 to 1,000xc2x0 C., its energy conversion efficiency is fairly higher than any other conventional fuel cell and has advantages of very low environmental pollution, requiring no fuel modifiers, and being capable of complex power generation.
Depending on their shapes, SOFC are clarified as tubular and planar. For tubular SOFC, air electrode-supported type fuel cells are developed in U.S.A. and Japan (see, S. C. Singhal, xe2x80x9cRecent progress in tubular solid oxide fuel cell technologyxe2x80x9d in Solid Oxide Fuel Cells V. PV 97-40 p. 37 (1997)). In the case of the planar SOFC, two types of configurations were followed: self-supported type in which electrolytes are used as supports and fuel electrode-supported type. (see, H. P. Buchkremer, U. Diekmann, L. G. J. de Haart, H. Kabs, D. Stover and I. C. Vinke, xe2x80x9cAdvances in Manufacturing and Operation of Anode Supported SOFC Cells and Stacksxe2x80x9d in Third European Fuel Cell Forum p. 143 (1998)).
Based on the advantage of conducting easy sealing and of being of high thermal resistance, the tubular SOFC are now under extensive study in many companies and research institutes, including Westinghouse. However, tubular fuel cells suffer from a disadvantage of being high in production cost because air electrode materials, such as La, Mn, etc., are very expensive.
In addition, another disadvantage of tubular fuel cells is that air electrodes have poor strength because the air electrodes are ceramics themselves compared to the fuel electrodes made of cermets composed of metal and ceramic. In addition, conventional air electrode-supported tubular SOFC are economically unfavorable because an Electrochemical Vapor Deposition (EVD) process, which accompanies high costs, usually conducts the coating of electrolyte on the air electrode support.
On the other hand, extensive attention is paid to electrode-supported type as they are found to be prepared at low cost with maintaining high mechanical strength.
Therefore, it is an object of the present invention to overcome the above problems and to provide a fuel electrode-supported tubular SOFC which is greatly improved in support strength without degradation of cell performance.
It is another object of the present invention to provide a method for manufacturing a fuel electrode-supported tubular SOFC with less production cost.
The fuel electrode-supported tubular SOFC has a structure in which the fuel electrode made of the metal (Ni)/ceramic (YSZ) cermets also play a role as a support. The electrolyte layer underlaying the air electrode is coated as a thin film on the support by a slurry coating process, which is a low-cost process.
In one embodiment of the present invention, there is provided a fuel electrode-supported tubular solid oxide fuel cell, comprising: a porous, tubular fuel electrode made of YSZ with a Ni content of 30-50 vol %, playing a role as a support; an electrolyte layer coated on the fuel electrode; and an air electrode made of (La, Sr)MnO, overlaying the electrolyte layer.
In another embodiment of the present invention, there is provided a method for manufacturing a fuel electrode-supported tubular solid oxide fuel cell, comprising the steps of mixing Y2O3-stabilized ZrO2 powder containing 30-50 vol % of Ni with a carbon powder as a pore forming agent. The slurry is molded into fuel electrode tube by extrusion and pre-sintered. The electrolyte slurry made of Y2O3-stabilized ZrO2 is coated on the pre-sintered fuel electrode tube by slurry coating process and sintered. Finally air electrode slurry made of (La, Sr)MnO3 powder is coated on the electrolyte layer in a slurry coating process. The fabrication process is completed in sintering the coated air electrode.